When preparing a polyurethane foam, it is desirable that the density of the polyurethane foam to be decreased, because the cost of preparing becomes less. However, the mechanical strength of the polyurethane foam is lowered when its density is decreased because the amount of a resin per unit volume is reduced.
In order to improve the mechanical strength of the polyurethane foam, there has been proposed to use additives such as a cross-linking agent, a polyfunctional polyol and an aromatic polyol together with the starting polyol.
The above proposal, however, has some problems. That is, the use of the cross-linking agent and the polyfunctional polyol in the preparation of an isocyanate prepolymer tends to cause gelation. Also, when a prepolymer which is made from the aromatic polyol is used as a starting material for a polyurethane foam, it is difficult to adjust the viscosity and melting point of the prepolymer to a level suitable for the preparation of a polyurethane foam because the aromatic polyol has a remarkably high viscosity and a high melting point.
One object of the present invention is to provide a process for preparing a polyurethane foam which has a low density, acceptable appearance and texture, and excellent mechanical strength, such as tensile strength and tear strength.
The above and other objects of the present invention will be apparent from the following description.